The present invention relates to a light sheild curtain assembly for a reciprocating optical scanning head of the type employed in an electrostatic copying machine.
In an electrostatic copying system, a scanning head is often employed which is reciprocatable adjacent to an original document for reproduction to expose a photoconductive member. Since the scanning head necessarily comprises a light source to illuminate the document, it is important that all light be prevented from reaching the photoconductive member except for that projected by the optical scanning system. For this reason, it is known to provide two rollers which extend perpendicular to the reciprocating direction of the scanning head and are disposed on opposite sides of the scanning head.
Two curtains are provided, each curtain being fixed at one end to and rolled around one of the respective rollers and being fixed at its other end to the scanning head. Spiral springs are associated with the rollers to urge the rollers to roll the curtains thereon. As the scanning head moves in one direction, it unwinds the curtain form one roller and the other curtain is rolled on the other roller by the respective spiral spring.
Whereas this prior art curtain arrangement is effective in shielding the photoconductive member from stray light, it suffers from several drawbacks. Since the springs must be wound and unwound by a great extent during each reciprocation of the scanning head, they have short service lives due to fatigue failure. They also lost their stiffness prior to failure, which tends to allow the curtains to slacken and lose their light shielding efficiency. Another cause of the springs losing their stiffness is that one of the springs must remain unwound while the copying apparatus is inoperative. If the springs are made initially stiffer to increase their service lives, there is the danger of tearing the curtains due to excessive tensile force coupled with the natural deterioration of the curtain material. In addition, such a spring loaded arrangement is relatively difficult to assemble.